The primary objectives of this study will be to determine the effects of baclofen on the reinforcing and subjective effects of oral methamphetamine in normal human volunteers. Baclofen, a GABAb agonist, is capable of decreasing a variety of neurotransmitters from being released in the mammalian CNS. It has also been shown that baclofen decreases the reinforcing efficacy of cocaine and methamphetamine in animal models of drug self-administration. In humans, baclofen is currently in clinical trials for the treatment of cocaine dependence. These studies show promising results, with decreased cocaine intake in an outpatient population. Cocaine and methamphetamine work by similar mechanisms to produce similar effects within the mammalian CNS. Methamphetamine use and abuse is a growing problem in the United States and worldwide. Since baclofen has not been used in clinical trials for methamphetamine dependence, the ability to test this theory in a controlled environment would provide a valuable line of evidence for baclofen as a clinical treatment of methamphetamine abuse. Using previously validated subjective measures and choice procedures, we will determine the effects of baclofen on methamphetamine's subjective and reinforcing effects in humans.